Method of heating solids in a pneumatic conveyer conduit



Dec. 4, 1962 w. KRONIG ET AL 3,066,927

METHOD OF HEATING SOLIDS IN A PNEUMATIC CONVEYER CONDUIT Filed March 25, 1960 ATTORNEY Unite States When conducting thermal or catalytic processes in which solids are conveyed in circulation, it is known that the solids which have given off some of their sensible heat to the reactants in a reaction chamber can be reheated during the circulation in a vertical conveyor, usually a vertical conveyor conduit by bringing them into contact with hot gases which at the same time constitute the conveying means in the vertical conveying conduit. Two methods have been developed on this principle:

One method consists in that the hot gases are generated in a combustion chamber disposed in front of the conveyor conduit and these gases are then used for conveying and heating the circulating solids. This process has the important disadvantage that it is necessary with a relative high heat consumption to employ very large combustion chambers and that these must moreover be made of highly refractory materials, since very high temperatures are set up in the combustion chambers, these temperatures frequently being higher than 2000 C.

in order to overcome this disadvantage, another procedure has been developed which consists in that the head section of the fuel supply means is displaced into the zone in which the conveying medium and the solids are mixed. The burner is also sometimes arranged just beneath the supply point for the fuel into the conveying conduit. This method has the advantage over the previously described method that the hot gases being set up at the burner are immediately cooled by the solids surrounding the fiame, in that the solids immediately take 1 up the heat and thus prevent the Wall surfaces being too strongly heated. This procedure has also proved satis' factory in commercial installations.

It has, however, been found that when using this procedure, the abrasion of the circulating solids is quite considerable, combined with an appreciable wear of the conveying conduit.

it has now been found that when solids are conveyed pneumatically in a vertically ascending conduit, with simultaneous heating of the solids to be conveyed by gases of combustion which are generated by means of a burner by combustion of fuels with oxygen-containing gases, the burner being arranged below the solids to be conveyed, it is advisable to work in such a Way that the spacing of the burner opening from the inlet point of the solid which is to be conveyed is about 1 to 5 times and advantageously 1.5 to 3 times the diameter of the conveyor conduit at the point where the solid substances are introduced. By the term burner opening is to be understood the upper portion of the head section of the fuel supply arrangement.

When using this burner arrangement, it is possible to lower the quantity of the conveying gases by about 20% as compared with the quantity which must be definitely maintained with the known process in order to prevent a disturbing falling back of the solid materials to be conveyed. With this procedure, an appreciable reduction of the abrasion by the solid material is achieved, probably because of the fact that a substantially uniform temperature and correspondingly approximately uniform fiow is set up at the inlet point of the solid materials, which cannot be achieved if the combustion only occurs above atent ICC or directly beneath the supply point of the solid material, since in such cases uncontrollable mixtures of cooler conveying air and hot combustion gases are obtained, that is to say, the flow is not uniform. Moreover, the said possibility of reducing the quantity of conveyor gas might contribute very substantially to the lowering of the abrasion.

On the other hand, the method according to the invention has the advantage as compared with the use of a precombustion chamber that large combustion chambers of highly refractory material are not required, since the space between the burner opening and the supply point of the solid material is not sufficient in order also to bring the walls to high temperatures.

By working in the said range as regards the spacing of the burner opening from the supply point of the solid materials, there are obtained decisive advantages over the prior known processes, these advantages consisting on the one hand in the great reduction of the abrasion and on the other hand in that it is possible to dispense with extensive combustion chambers. The reduction of the abrasion of the heat carrier, for example sand, which is produced according to the invention is possibly also to be attributed in part to the fact that the heat carrier is now not directly subjected to the action of the flames and thus is given a smaller thermal shock. However, quite apart from this, there is also a reduction in the Wear on the conveying conduit.

The spacing within the above-mentioned range which is chosen in each individual case depends inter alia on the nature of the burner. If a burner is used which has a relatively small opening by comparison with the diameter of the conveying conduit, the spacing of the burner opening from the supply point of the solid material will be brought closer to the upper limit of the said range. If a burner is used which burns on a relatively large surface, for example, if the burner consists of several openings, the Spacing between the burner opening and the supply point of the solid material Will be brought closer to the lower limit of the range.

The invention will be further described with reference to the accompanying drawing forming a part of this specification and illustrating by way of example a portion of an apparatus for carrying out the process of the invention.

Referring to the drawing, F represents the conveying conduit through which the conveying gas, as for example air, is passed upwardly. The solid heat-carrier particles are introduced into the conduit F through the pipe C. A burner B provided with a burner mouth BM is arranged in the conduit F below the inlet point for the solid particles from the pipe C into the conveying conduit F. The burner mouth is arranged in the conduit so that the distance A from the burner mouth to the inlet point of the solid particles into the conveying conduit is from 1 to 5 times the diameter D of the conveying conduit.

The following example further illustrates the present invention without limiting it thereto.

Example A stream of sand granules with the size 0.5 to 1.2 mm. is circulating in a system which consists of a vertical conveying conduit, a supply container and a reaction chamber. The sand granules are conveyed upwardly by pneumatic means in the conveying conduit and at the same time are heated to a temperature of approximately 825 C. By way of the supply container, the heated sand reaches the reactor in which it gives off some of its sensible heat to the reactants in order then to return at a temperature of about 700 C. into the conveying conduit. The supply of the sand into the conveying conduit is efiected in a concentric annular manner from slots. About 25 tons of sand per hour are circulated with an input of a quantity of conveying air of 1900 standard cubic meters per hour. The spacing of the burner opening from the supply point of the sand was in this case 0.2 times the diameter of the conveying conduit at the sand supply point. The hourly abrasion of sand was in the region of 40 kg. Furthermore, with the installation in operation for a long time, a clear indication of wear was found in the conveying conduit.

On the other hand, if the burner, under otherwise identical conditions, is disposed at such a lower level that the spacing of the burner opening from the sand supply point is 2.5 times the diameter of the conveying conduit at the said supply point, the hourly quantity of conveying air could be reduced to 1500 standard cubic meters without impairing the uniformity of the sand supply, whereas it was not possible with the firse-mentioned arrangement to lower this quantity to below 1900 meters without the sand supply being interrupted. The sand abrasion was reduced to 20 kg./hour and the wear in the rising pipe decreased appreciably.

Similar good results are also obtained if other heat carriers are used which can be also catalytically active, especially such heat carriers which tend to have interior tensions upon heating which cause a crumbling to pieces of said carriers if they are subsequently subjected to mechanical stresses. Examples of such heat carriers are ot-aluminum oxide or such substances as aluminum oxide gel or silica gel.

We claim:

1. Method of transferring heat which comprises introducing solid divided heat-carrier particles into an ascending gas stream and pneumatically conveying the same upwardly in said stream, heating said particles with hot combustion gases without change in their physical structure by introducing the hot combustion gases as a separate stream into said ascending gas stream at a point below the point of introduction of the particles and spaced therefrom one to five times the width of the ascending gas stream at the point of introduction of the solid particles, transferring the heated particles to a zone to give up a portion of their sensible heat and recycling the same to said ascending gas stream.

2. Method according to claim 1 in which said hot combustion gases are introduced into said ascending gas stream at a point below the point of introduction of the particles and spaced therefrom 1.5 to 3 times the width of the ascending gas stream at the point of introduction of the solid particles.

References Cited in the tile of this patent UNITED STATES PATENTS 2,572,484 Howle et al Oct. 23, 1951 2,612,263 Slavick Sept. 30, 1952 2,621,034 Stecker Dec. 9, 1952 2,746,735 Bradford May 22, 1956 2,809,023 Schoenmakers et al. Oct. 8, 19 57 

